A shard in elasticsearch is a Lucene index, and a Lucene index is broken down
into segments. Segments are internal storage elements in the index where the
index data is stored, and are immutable. Smaller segments are periodically
merged into larger segments to keep the index size at bay and to expunge
deletes.

The merge process uses auto-throttling to balance the use of hardware
resources between merging and other activities like search.

The merge scheduler (ConcurrentMergeScheduler) controls the execution of merge
operations when they are needed. Merges run in separate threads, and when the
maximum number of threads is reached, further merges will wait until a merge
thread becomes available.

The merge scheduler supports the following dynamic setting:

index.merge.scheduler.max_thread_count

The maximum number of threads on a single shard that may be merging at once.
Defaults to
Math.max(1, Math.min(4, Runtime.getRuntime().availableProcessors() / 2))
which works well for a good solid-state-disk (SSD). If your index is on
spinning platter drives instead, decrease this to 1.